A spoked wheel for a bicycle comprises a peripheral crown or rim, a central hub and a plurality of spokes extending between the hub and the rim. The tension of the spokes are usually adjustable at the hub and/or at the rim. This tension is usually adjusted by providing threading at one end of the spokes and a threaded hole attachment at the hub or the rim, respectively. The second end of the spokes can be provided with a second threading or a head, for example rectangular or circular, housed in a suitable seat, for example hooked in a hub flange or inserted in a bayonet coupling at the rim, respectively.
Rims having a channel, with fins where the beads of the tire are coupled and at the bottom of which a plurality of holes are made, are known. The holes receive a spoke attachment element provided with a hole with a shaped seat or with a threaded hole. The length of the hole threading of the spoke attachment element can be equal to the spoke threading, or else longer to also allow the tension adjustment of the spoke by varying the screwing-in depth of the spoke in the threaded hole of the spoke attachment element. By way of terminology, the spoke attachment elements are also called “nipples”, if they allow tension adjustment of the spoke, or else “barrels” if they do not allow this.
Single channel rims have low structural stiffness and therefore greater deformability from lateral loads. Moreover, such single channel rims are not suitable for use with tubeless tires since the coupling channel is not airtight.
To increase structural stiffness, rims having two chambers, a circumferentially external tire coupling chamber and a circumferentially internal spoke attachment chamber, are also known. More specifically, such rims are made up of two side walls connected or filleted at one end by a circumferentially internal wall or lower bridge and at an intermediate point by a circumferentially external wall or upper bridge or partition, so as to have a substantially inverted A-shaped cross-section. The circumferentially external portions of the side walls, typically provided with a border, form the tire coupling channel with the upper bridge or partition, whereas the circumferentially internal portions of the side walls form, with the lower bridge and the upper bridge, a spoke attachment chamber.
Instead of a single chamber, two or more chambers can be provided through one or more partition walls, extending substantially parallel to the upper bridge or substantially transversal to it. Each wall (upper bridge, lower bridge, side and partition walls) can also be shaped in various ways, forming a complex cross section of the rim.
Rims of this type can be manufactured by casting of carbon fiber (see for example the European patent application No. 02002949.9 and publication No. 1,231,077 A2). Alternatively, such rims are manufactured by extrusion of an aluminum profiled bar. The profiled bar is shaped as a circle usually by calendaring and its terminal ends are joined by welding, for example flash welding or TIG welding, by cementing after insertion of a metallic sleeve, by pin-connection or otherwise.
With these types of rims, the problem is how to attach spokes in respective spoke attachment seats provided in the spoke attachment chamber(s), using the aforementioned spoke attachment elements.
In a first solution, described in U.S. Pat. No. 5,975,646 assigned to Campagnolo S.r.l., this is carried out by forming a plurality of access openings in the upper bridge, each at a respective spoke attachment seat. Such access openings allow insertion of the spoke attachment elements (nipples or barrels) and of the tool for screwing them onto the spokes or for holding them during the attachment of the spokes. To preserve a suitable structural stiffness of the rim, the upper bridge must however be thick, with a consequent undesirable weight increase of the rim. Moreover, such a rim is not suitable for tubeless tires since the tire coupling channel is not airtight because the aforementioned access openings are not airtight.
U.S. Pat. No. 6,474,746 assigned to Campagnolo S.r.l., avoids this and provides a rim in which the upper bridge has just a hole for the inflation valve for the tire and therefore is sufficiently rigid and light, as well as suitable for use with tubeless tires. The reference proposes to use supports having a seat for supporting a respective nipple at least partially outside of the rim section. Each nipple support comprises a substantially U-shaped bracket having the seat and hung on a pivot extending transversally between a pair of non-threaded holes formed in the side walls of the circumferentially internal chamber, or in flanges protruding from the side walls of the circumferentially internal chamber. Regarding such a solution, besides the nipples, additional elements are necessary, with consequent additional assembly steps. The solution also creates weak points in the rim.
Other known solutions for providing a rim in which the upper bridge has just the hole for the inflation valve of the tire and therefore sufficiently rigid and light, as well as suitable for use with tubeless tires, are described in European patent applications EP 0 896 886 A1, EP 1 101 631 A1, EP 1 167 078 A1, all assigned to Mavic S.A. In all three references, the spoke attachment seats are threaded holes in which a threaded end of a spoke, a spoke attachment element such as a nipple or a barrel, or an insert in which to insert a nipple or a barrel are screwed from the outside of the chamber. Since the thickness of the rim walls must be small for weight reasons, it is necessary to form the threaded holes by material deformation, so that a “chimney” is created in which a threading of the required length can be formed. Regarding these solutions, they are complex and expensive since forming the threaded holes in the lower bridge or in any case in the walls of the circumferentially internal chamber is a difficult and complex process.
Therefore, a simple and cost-effective solution to manufacture a spoked wheel for a bicycle, sufficiently strong while still being light, is still desirable.